What is the difference between 'Unlink' and 'Delete' in the Outliner?
Unlink removes objects from collections, Delete erases them permanently.

Removing Data

Unlinking doesn't remove the object or its geometry from the opened file, it just removes them from the collection clicked on. If an object is in more than one collection simultaneously it will still remain in others, otherwise it may become Orphaned Data.

Orphan data is "garbage collected" and may be removed on saving file.

Put Outliner in Orphan Data Mode to check before saving to avoid losing any important data. 

Deleting fully erases the object and its geometry immediately, while also removing it from all the collections it was in, from the scene and from the file, reducing file size and memory consumption.